1. Field of the Invention
The present invention relates to a transformer operation control apparatus and method for suppressing a magnetizing inrush current that transiently passes through a transformer, when the transformer is connected to an electric power system.
2. Description of the Background Art
For suppressing a magnetizing inrush current, it is important to control the timing for applying a current to the transformer.
Japanese Patent Laying-Open No. 2004-208394 discloses the following technique. A magnetic flux signal is calculated by integrating the transformer-side voltage of the breaker for each phase. A difference between a magnetic flux value at the time point where the transformer-side voltage has converged to zero value after input of an open command, and a center value of the magnetic flux signal before the input of the open command is calculated as a residual flux. Then, the phase where the absolute value of the residual flux is greatest is set as a first closing phase. As to the timing of applying the current of the first closing phase, a timing when an apply magnetic flux difference, which is an absolute value of the maximum difference between the steady magnetic flux value and the residual flux value, is greatest is calculated based on the residual flux of the first closing phase and the pre-arc characteristics and closing time variation characteristics of the three-phase breaker obtained in advance, and set as a target apply time point of the first closing phase.
As to the applying timing for the currents of the other two phases, a timing when the apply magnetic flux difference is smallest when residual flux is 0 is calculated, based on the pre-arc characteristics and closing time variation characteristics of the three-phase breaker obtained in advance, and set as a target apply time point of the other two phases. Then, the sum of the time from the reference point until the target apply time point of the two other phases and a delay time corresponding to an integral multiple of a cycle of the three-phase power supply having been set in advance is set as a target apply time of the other two phases.
Japanese Patent Laying-Open No. 2006-040566 discloses the following technique. Each contact in each arc-extinguishing chamber of a breaker is opened simultaneously for the three phases at the voltage zero point of one of the three phases, whereby currents are shut off. Then, setting the one phase opened at the voltage zero point as a first apply phase, and taking into consideration of a residual flux of the transformer, the optimum time point where a magnetizing inrush current at the time of current application is smallest is set as a target apply time point of the first apply phase. Herein, the target apply time point of the first apply phase is the time point when the residual flux of the one phase of the transformer agrees with the steady magnetic flux. As to the applying timing for the currents of the other two phases, the voltage zero point of the first apply phase at arbitrary time point after the closing time point of the first apply phase is set as a target apply time point of the second and third apply phases.
According to the conventional techniques, the apply timing of the first apply phase in which a current is applied firstly among the three phases may deviate from the target apply time point, because of variations in the closing operation time of the breaker or variations in the pre-arc time. In such a case, a magnetic flux by the current passing in the first apply phase is superimposed on a residual direct current component magnetic flux. Then, transient magnetic saturation occurs and a magnetizing inrush current passes. In this case, as to the other two phases, even if a current is applied at the timing where the voltage of the first apply phase becomes 0, magnetizing inrush currents also transiently pass in the other two phases, which are hostile to the transformer.